The anti-cancer effects of cytosine arabinoside (ARA-C) are well known. However, effects on nonmalignant cells have not been elucidated and may be important to understanding treatment-related toxicity. The purpose of this study was to examine the effect of ARA-C on nondividing vascular endothelial cells. The objectives were to determine the effects of ARA-C on cell viability and to ascertain whether ARA-C caused apoptosis in cultured vascular endothelial cells and hydrocortisone blunted caspase-3-induced apoptosis. Endothelial cells were cultured until confluent and mitotically quiescent then exposed to ARA-C (10(-7)to 10(-3) M) for 1 to 4 days. Some experiments involved cotreatment with hydrocortisone (10(-11),10(-10),10(-4), and 10(-3) M). Light microscopy and the colorimetric MTS assay were used to measure viability. Fluorescent annexin-V and DNA fragmentation assays were used to measure apoptosis, and a fluorescence-based enzymatic assay was used to measure caspase-3 activity, which is one pathway involved in the apoptosis cascade. Two-way ANOVA or the appropriate nonparametric test was used to determine statistical significance in studies of viability and apoptosis. Oneway ANOVA was used to determine statistical significance for caspase-3 activity. Viability was decreased with higher concentrations of ARA-C and increased days of treatment. The percentage of apoptotic cells increased with higher concentrations of ARA-C and increased days of treatment. ARA-C-treated samples showed DNA fragmentation, indicative of apoptosis. Caspase-3 activity increased after ARA-C addition; hydrocortisone blunted this increase. ARA-C caused apoptosis in nondividing endothelial cells in culture. Hydrocortisone may protect against ARA-C-induced apoptosis by reducing caspase-3 activity.